In laying concrete floors, ramps, pavement and like horizontal structures, it is customary in some instances to first provide a foundation or base slab, after which a top or finish layer is applied to the slab. Elongated boards or rods are mounted on the base slab to provide guide surfaces for the usual strike board or screed used in leveling or flattening the top surface of the finish layer. In order that the top surface be uniform over a fairly large area, it is important that the top surfaces of the guide boards or rods be disposed at a predetermined level from end to end thereof and relative to each other. This has heretofore been a problem, particularly when the top surface of a base slab is rough and uneven, requiring wedging of the guide boards or rods to bring the same, or at least portions thereof, to the required level.